Dual intermediate frequency amplifier circuit



Feb. 17, 1942. e. MOUNTJOY 2,273,134

DUAL INTERMEDIATE FREQUENCY AMPLIFIER CIRCUIT Filed Jan. 7, 1941 AAA v v v v vv ZINVENTOR ATTORNEY Patented Feb. 17, 1942 UNETE TES PATENT OFFICE DUAL INTERMEDIATE FREQUENCY AMPLIFIER CIRCUIT GarrardMountjoy, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Claims.

My present invention relates to radio receivers adapted to operate in signal ranges assigned to frequency modulation reception and amplitude modulation reception, and more particularly to superheterodyne receivers adapted for operating selectively to receive both amplitude and frequency modulated carrier waves.

At the present time the tendency is to design frequency modulation receivers in such a manner that they are also capable of receiving amplitude modulated carrier waves upon proper adjustment. As is well known, the 43 to 50 megacycle range is assigned to frequency modulation reception, while the standard amplitude modulation broadcast range covers a band of 550 to 1700 kilocycles. Because of the wide difference in signal frequencies between the amplitude and frequency modulation bands two different intermediate frequency values are necessary, since the universal type of receiver employed for reception at this time is the superheterodyne receiver. The intermediate frequency amplifier system which is employed for this dual purpose should be capable of providing extremely high gain with good stability, and satisfactory reduction of image interference. For the frequency modulation band an intermediate frequency of between 2 to 4 megacycles can be utilized, and most commonly a frequency of 4.3 megacycles seems to be most satisfactory. For amplitude modulation reception a value of 455 kilocycles is commonly used.

It may, therefore, be stated that itis one of the main objects of my present invention to provide a dual purpose intermediate frequency amplifier system adapted for use in a superheterodyne receiver which may be operated in either of the ranges assigned to frequency modulation and amplitude modulation reception.

Other objects of the invention are to improve the aforesaid type of dual purpose intermediate frequency amplifier network, and to provide one which will operate with efiiciency for amplitude or frequency modulation reception, and yet be economical to manufacture and assemble.

The novel features which I believe to be characteristic of my invention are set'forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagramatically a circuit organization whereby my invention may be carried intoeffect.

Referring now to the accompanying drawing; there is shown thecircuit details of the intermediate frequency amplifier network of a superheterodyne receiver adapted for service in either the amplitude modulation reception band or in the frequency modulation reception range. It is not believed necessary to show the circuit details prior to the converter output circuit, since those skilled in the art are fully acquainted with the various methods which can be employed to feed a converter with signal frequencies in either of the desired reception bands. For example, if the converter is of the combined local oscillatorfirst detector type, then the local oscillator network; and the signal input network will be designedso. that they can be readily and concurrently. switched over from joint operation in the higher 43 to megacycle band to joint operation through the lower frequency (550 to 1700 kilocycle) band. Hence, it will be assumed that the rectangle denoted as converter is fed with signal and local oscillation frequencies so asto produce in the output circuit thereof intermedi-' ate frequency energy having a center frequency of 4.3 megacyclesin the case of frequency modulation reception, or intermediate frequency energy of- 455 k-ilocycles in the case of amplitudev modulation reception.

For this purposethere are arranged in series in the converter output circuit a pair of resonant circuits each tuned to a different intermediate value. Thus, the coil l is the primary winding of the transformer T1, and has connected in shunt thereacross a condenser 2 which resonates it to the center frequency of the frequency modulated signals, and the latter center frequency is 4.3 n1egacycles. The low potential end of the circuit |-2 is connected by lead 3 to a second parallel resonant circuit comprising the primary winding-4' which is shunted by condenser 5. The circuit 4-5 is tuned to the intermediate frequency of 455 kilocycles. ofcircuit 54-is-connected to the B+ terminalof the power supply source through a voltage re ducing resistor 6, and the resistor 6 may be by.-. passed to ground by condenser I.

The first intermediate frequency amplifier. tube. is designated by numeral 8, and the latter is shown as a pentode which may be of the SAC? type. The cathode of this tube is connected to ground through the usual self-biasing network 9. The signal control grid thereof is connected to the high potential side of the secondary winding ll? of transformer T1. The coil It) is shunted by a tuning condenser II. One terminal. of the condenser ll is'connected to the high potential end ofwinding l0, while theopposite' end iscon- The low potential end nected to the low potential end of winding l through a path which includes the lead l2, the coil 3, lead I l and the adjustable tap l which cooperates with the fixed contact point IS.

The coil 13 is magnetically coupled to the coil 4. There is shunted across coil |3 a condenser H which tunes coil l3 to the intermediate frequency of 455 kilocycles. Hence, it will be seen that the networks 54 and |3--|'| provide the usual double tuned intermediate. frequency transformer of a standard broadcast receiver of the superheterodyne type. The condenser I| tunes coil |0 to the center frequency of 4.3 megacycles. The grid leak resistor is connected between the signalgrid of tube 8 and the low potential side of condenser The low potential end of coil I0 is connected to a second fixed contact point I6 through a condenser CT. When tap I5 is connected to contact l6 then frequency modulated signals having a center frequency of 4.3

megacycles will be transmitted through the intermediate frequency amplifier network. However, when tap I5 is adjusted to contact I6 then the network is connected to receive the intermediate frequency energy of 455 kilocycles.

The plate of tube. 8 is connected to the +250 volts point on the direct current supply source through a path which includes a coil T2. A condenser 2| is connected in shunt with coil T2, and a resistor 22 is connected in shunt with the coil and with the condenser 2|. The lead 23 connects the low potential terminal of coil T2 to an intermediatepoint on coil 24, the condenser 25 resonating coil 24 to the intermediate frequency of 455 kilocycles. to reduce the gain at 455 kilocycles. The condenser 2| tunes the coil T2 to the center frequency of 4.3 megacycles. A voltage reducing resistor 26 may be inserted between the plate voltage supply of tube 8 and the screen lead of the tube.

The plate terminal of coil T2 is connected through coupling condenser 30 to the signal input grid of the second intermediate amplifier tube, which also may be of the BAC? type. The cathode circuit of the tube 3| includes a self-biasing network 32, whileits plate circuit includes a pair of parallel resonant circuits arranged in series in very much thesame manner as the plate circuit of the converter tube. Thus, the transformer Tshas its primary winding shunted by the condenser 4|, and circuit 40- is tuned to the 4.3 megacycle frequency. The amplitude modulation transformer has its primary resonant circuit tuned to the 455 kilocycle value, while its resonant secondary circuit 5| is also tuned to the same intermediate frequency value. The positive potential for the plate of tube 3|' is supplied through the coils of the resonant primary circuits which are shown arranged in series relation. The secondary resonant circuit of transformer T3 is designated by numeral 60, and is tuned to the center frequency value of 4.3 megacycles. The designation FM is placed over each of transformers T1, T2 and T3 to denote that these are the frequency modulation signal transmission Paths. Similarly, there is disposed the letters AM over each of the transformers which transmit the amplitude modulated signals of intermediate frequency value.

The frequency modulated voltage developed across T3 is fed to any desired type of amplitude modulation limiter circuit. It is not believed necessary to describe the details of such a circuit.

The coil 24 is tapped as shown 7 It is believed sufiicient to point out that the limiter may be of the usual type wherein there is employed a grid circuit utilizing substantially zero bias and which acts in the manner of an easily overloaded amplifier tube. The purpose of the limiter is, of course, to eliminate amplitude Variations in the frequency modulated signals. Such variation may be produced by noise impulses, fading or the passage of the frequency modulated signals through the preceding tuned circuits. Subsequent to limiting action the frequency modulated signals will be detected in any well known type of frequency modulation detector, and the detected energy would then be transmitted through an audio amplifier.

As far as the amplitude modulation signals are concerned, the voltage developed across circuit 5| may be impressed upon the diode section of a multiple duty tube, say one of the 68627 type. The high potential side of condenser 5| is shown connected to the commonly-connected anodes. The low potential end of the circuit is connected to the grounded cathode through the load resistor ID, the latter being by-passed for intermediate frequencies by condenser The audio voltage developed across resistor 10 may be transmitted to the aforementioned audio network. The direct current voltage component of the rectified voltage developed across resistor 10 may be utilized for automatic volume control action, and the leads designated by the letters AVC are to be understood as the automatic volume control connections to the intermediate frequency amplifier tubes 8 and 3|.

Thus, the anode end of load resistor 10 is connected to the signal grid of tube 3| through a path which includes lead and filter'resistors 8| and 82 arranged in series relation. The low potential end of coil I3 may be connected through 407 lead to the junction of resistors 82 and 8|,

and the lead -90 may be bypassed to ground by condenser 9| in order to provide proper elimination of pulsating components in the direct current voltage employed for automatic volume control action. The action of the automatic volume control circuit is well known. In general, and during amplitude modulation reception, the control circuit functions to vary the gain of each of amplifier tubes 8 and 3| in a sense to compensate for carrier amplitude variation at the signal collector of the receiving system.

Only one service, either AM at 455 kilocycles or FM at 4.3 megacycles, is desired at a time. Response at the unwanted frequency is reduced by the switching arrangement |5|6 l6. Reducing the unwanted frequency gain in. the first transformer prevents cross-talk in the succeeding tube. Once the gain is reduced, then straight amplification can be used (as in T2T3) without further switching arrangements.

The amplifier of the invention has two stages of intermediate amplifier amplification to obtain high gain in the FM section. The AM section likewise consists of two stages. One stage would be adequate for gain, but the second stage required for FM can be economically utilized to obtain better selectivity shape and more perfect AVC" action desirable for the AM- channel. The converter and amplifier tubes 3 and Marc used for both services. Separate detector systems adequately restrict the gain in the last stage to their respective sections so that no appreciable amplification is exhibited by the last stage for 455- kilocycles in the FM section nor at 4.3 megacycles in the AM path. The first two transformers are required to pass both frequencies in series, therefore a switch section is necessary to reduce gain at the unwanted frequency. 'This operation is most effective in the first transformer T1 than in the second, as the overload of the first intermediate frequency anoplifier. tube is thus prevented.

The gain of theconverter and second stage 3| are made large compared to the first amplifier 8 for reasons of stability. This is done by making the impedancesofTi and T3 high relative that of T2. Since the FMAM switch is not used on the second stage this distribution of gain is of further advantage in reducing interference. In AM reception it may occur that the fundamental o'r'harmonics of the. local oscillator fall at 4.3 megacycles. In this case considerable attenuation of the switch section in the process of opening the secondary of the first transformer reduces the. gain of the converter stage at 4.3 megacycles from 39. to 0.12. If this reduction should prove inadequate, the trap capacitor CT is useclto tune the secondary inductance NJ to 4.3 megacycles to form a series tuned shunt across the grid of amplifier 8. The capacitor may be either fixed or variable. Gain at 455' kilocycles through the entire FM section is 7.3 times thereby. producing a ratio of 4.3 megacycles to 455 kilocycles response of 12,000 times.

Gains measured for desired signal at various stages were:

FM gain AM gain (at 4.3 mo.)

Stage (at 455 kc.)

Overall The performances indicated may be considered satisfactory for many services. Further increase in selectivity may be had be converting the second transformer from a single circuit to double circuit resonance in either or both sections. By this'means extra rejection for adjacent channels may be had without impairing the frequency response of the desired channel. Reduction of coupling may accomplish considerable increase in cost but at the sacrifice of wide frequency response in the desired channel. In the AM section of transformer T2, a solid wire winding with a Q of 50 was used for economy and to preserve the broad-topped resonant response of the two-circuit transformers T1 and T3. The use of a Litz winding of higher Q would improve selectivity somewhat, but it is suggested that this means be not employed, as the tuning shape would be made unduly sharp for the slight gain so desired. All adjustments of the FM elements .are assumed to be accomplished from the top of the chassis, while the AM adjustments are performed from beneath the chassis. Other mechanical arrangements will suggest themselves as alternates to those skilled in the art. For example, all adjustments could be arranged to be available from the top of the chassis, and at opposite sides of the transformer shields.

The arrows through the coils designate iron tuning slugs or cores which may be adjusted to vary coil coupling percentage. Tubes of the 6AC7 type were used because of their relatively high Gm. If tubes of lower Gm are used gain may be restored at least in part in the FM section by increase in inductance of the second transformer. The limitation of increase will be determined by the degree of regeneration encountered. In the AM section regeneration is no problem whatever, and any usablegain may be obtained by selecting the tapped-downlpoint of the second transformer to suit the service intended and the tubes selected for use. With the GAG? tubes a 455 kilocycle gain of some 20,000 times was arbitrarily derived. This should be quite adequate for receivers which do not use a stage of radio frequency amplification. For receivers which do, much less gain would be desirable. I

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a plurality of cascaded amplifier tubes, an output circuit for the first of said tubes consisting of at least two series connected resonant circuits, said circuits being tuned to different frequencies, the second of said tubes having an input circuit comprising a first resonant circuit tuned to the same frequency as one of said series circuits and reactively coupled thereto, a second resonant circuit tuned to the frequency of the second series circuit and reactively coupled thereto, said second resonant circuit being included in said first resonant circuit, and means for connecting a. capacity in circuit with said first resonant circuit to provide a rejection trap for high frequency energy of said first resonant circuit frequency.

2. In combination with a plurality of cascaded amplifier tubes, an output circuit for the first of said tubes consisting of at least two series connected resonant circuits, said circuits being tuned to different frequencies, the second of said tubes having an input circuit comprising a first resonant circuit tuned to the same frequency as one of said series circuits and reactively coupled thereto, a second resonant circuit tuned to the frequency of the second series circuit and reactively coupled thereto, said, second resonant circuit being included in said first resonant circuit, said second tube having an output network comprising a pair of series connected output circuits, one output circuit being tuned to said first series circuit frequency, and the second output circuit being tuned to the second frequency, and both second tube output circuits having a relatively small impedance value to minimize regenerative effects.

3. In combination with a plurality of cascaded amplifier tubes, an output circuit for the first of said tubes consisting of at least two series connected resonant circuits, said circuits being tuned to different frequencies, the second of said tubes having an input circuit comprising a first resonant circuit tuned to the same frequency as one of said series circuits and reactively coupled thereto, a second resonant circuit tuned to the frequency of the second series circuit and reactively coupled thereto, said second resonant circuit being included in said first resonant circuit, said first resonant circuit consisting of a coil shunted by a condenser, and said second resonant circuit being in series relation with said coil and condenser.

4. In a superheterodyne receiver of the type comprising a converter adapted to have frequency modulated carrier signals and amplitude modulated carrier signals selectively impressed thereon, and the carriers being of widely difierent frequencies; the improvement which comprises at least one intermediate frequency amplifier tube, a pair of resonant circuits arranged in series in the output of said converter, one of said resonant circuits being tuned to a high intermediate frequency adapted to be used for frequency modulation reception, the second resonant circuit being tuned to a low intermediate frequency adapted to be used for amplitude modulation reception, a resonant input circuit for the amplifier tube tuned to said high intermediate frequency and coupled to said one resonant circuit, a second resonant input circuit for said amplifier tuned to the low intermediate frequency and coupled to the said converter second resonant circuit, and means for selectively closing said first resonant input circuit through said second input circuit to transmit said high intermediate frequency energy to said amplifier tube.

5. In a superheterodyne receiver of the type comprising a converter adapted to have frequency modulated carrier signals and amplitude modulated carrier signals selectively impressed thereon, and the carriers being of widely difierent frequencies; the improvement which comprises at least one intermediate frequency amplifier tube, a pair of resonant circuits arranged in series in the output of said converter, one of said resonant circuits being tuned to a high intermediate frequency adapted to be used for frequency modulation reception, the second resonant circuit being tuned to a low intermediate frequency adapted to be used for amplitude modulation reception, a resonant input circuit for the amplifier tube tuned to said high intermediate frequency and coupled to said one resonant circuit, a second resonant input circuit for said amplifier tuned to the low intermediate frequency and coupled to the said converter second resonant circuit, and means for selectively closing said first resonant input circuit through said second input circuit to transmit said high intermediate frequency energy to said amplifier tube, a second amplifier tube, a network coupling the tubes in cascade, said network comprising a pair of tuned circuits arranged in series relation in the first tube output, one of said pair being tuned to the high intermediate frequency, the second of the pair being tuned to the low intermediate, and said pair of circuits being of relatively low impedance compared to the corresponding resonant circuits between the converter and first tube.

GARRARD MOUNTJOY. 

